the apparent change of frequency of sound waves or light waves, varying with the relative velocity of the source and the observer: if the source and observer are drawing closer together, the observed frequency is higher than the emitted frequency
after C. Doppler (1803-53), Austrian mathematician and physicist
A change in the observed frequency of a wave, as of sound or light, occurring when the source and observer are in motion relative to each other, with the frequency increasing when the source and observer approach each other and decreasing when they move apart. The motion of the source causes a real shift in frequency of the wave, while the motion of the observer produces only an apparent shift in frequency. Also called Doppler shift.
The phenomenon by which the observed frequency of a wave changes as a result of a time change in the effective length of the path of propagation between the source of the wave and the point of observation. If there is a source of wave energy and an observer of the wave energy, the frequency of the waveform increases as the observer moves closer to the source, the source moves closer to the observer, or both.The frequency of the waveform decreases as the observer and source move farther apart. The phenomenon applies to all waveforms, including acoustical and electromagnetic waveforms. In acoustics, the pitch of the sound increases as the observer and source move closer together, and decreases as they move farther apart, as you may have noticed when listening to a train whistle as the train comes closer and then goes farther away. The combination of the Doppler effect and that of multipath fading causes the wooooo-wooooo sound. In telecommunications, the Doppler effect creates difficulties when a mobile device, such as a cellular telephone moves towards or away from a fixed base station at a high rate of speed.The Doppler effect is used in some forms of radar to determine the speed and direction of a moving object.The Doppler effect was first hypothesized by Johann Christian Andreas Doppler (1803
The change in electromagnetic frequency that occurs when the source of the radiation and its observer move toward or away from each other. The faster they come together, the higher the frequency. The faster they move away, the lower the frequency. Discovered by Austrian physicist Christian Doppler (1803-1853), this condition has a great effect on low-earth orbit (LEO) satellites as they weave towards and away from the earth. See Doppler radar.
A phenomenon observed with waves. The frequency of a wave of light or sound seems higher if the source is moving toward the observer and seems lower if the source is moving away. For example, if an automobile blows its horn as it travels past someone, the apparent pitch of the sound will be higher as it approaches the person and then will grow lower as it passes and moves away.
The red shift of distant galaxies is a result of the Doppler effect on light.
An apparent change in the frequency of waves, as of sound or light, occurring when the source and observer are in motion relative to each other, with the frequency increasing when the source and observer approach each other and decreasing when they move apart.
The difference between the frequency of a wave (as of sound or light) as measured at its source and as measured by an observer in relative motion. The Doppler effect can be used to determine the relative speed of an object by bouncing a wave (usually a radar wave) off the object and measuring the shift in the frequency of the wave. This technique is the basis of Doppler radar, as used in traffic control and navigation systems. The Doppler effect is also known as the Doppler shift. ♦ If the source and the observer are getting farther apart, the observed frequency is lower than the source frequency. In the case of light waves, the phenomenon is known as red shift . The amount of red shift in the spectra of stars is used in astronomy to determine how quickly the Earth and those stars are moving apart. ♦ If the source and the observer are getting closer together, the observed frequency is higher than the source frequency. In the case of light waves, the phenomenon is known as blue shift .
A Closer Look The whistle of an approaching train has a higher pitch as the train approaches than when it recedes, even though that same whistle, heard by a passenger on the train, maintains a constant pitch. This is an example of the Doppler effect, common to all wave phenomena (in this case, a sound wave). Motion toward the source of a wave (or, equivalently, motion of the source toward the observer) entails that the peaks and troughs of the wave are encountered more quickly than if there were no motion, so the frequency of the wave is higher for the moving observer (hence the higher whistle pitch). Similarly, motion away from the source entails following the wave's motion, so the peaks and troughs are encountered less often, and the frequency is lower for the moving observer (hence the lower whistle pitch). The Doppler effect on light waves has enabled scientists to determine that the universe is expanding. The frequencies of light given off by various substances (such as the burning of hydrogen in the fusion reactions of most stars) has been found to be lower in distant galaxies and other celestial objects, a phenomenon called red shift, since the visible light is shifted toward the red, low-frequency end of the spectrum. Astronomer Edwin Hubble reasoned that the red shift was due to the Doppler effect. As galaxies speed away from us, the frequency of the light emitted appears lower. Doppler radar and sonar use the Doppler effect on reflected radio and sound waves to distinguish between stationary and moving objects and to determine the velocity of moving ones; the echolocation of bats and some whales also exploits the Doppler effect on reflected sound waves for navigating and catching prey.
As a motorcycle speeds forward, the frequency (and pitch) of the sound waves in front of the motorcycle become higher, and the frequency (and pitch) of the sound waves behind it become lower.